Glow type thermal switch



April 3, 1956 w. H. LAKE snow TYPE THERMAL SWITCH Filed Nov. 29, 1954 Fig.

LAGG/NG LAMP PERCENT Alma/v 7'0 Nsa/v [/v ICILL Gas Inven lror: WILLIam H. Lake, by W 6- 64 His A t torneg.

United States Patent GLOW TYPE THERMAL SWITCH William H. Lake, Chagrin Falis, Ohio, assignor to General Electric Company, a corporation of New York Application November 29, 1954, Serial No. 471,562- Claims. (Cl. 200-1135) The; present invention relates to thermal switches of the type wherein a thermally responsive element is actuated by heat generated by a gaseous electric discharge.

Switches of this type, known commercially asglow switches, comprise an hermetically sealed small glass envelope containing an ionizable gaseous atmosphere and a pair of normally spaced apart electrodes, at. least one of which is a thermally responsive bimetallic electrode. The pressure of the gas, the spacing of the electrodes and the characteristics of the other electrically conducting elements of the switch, are selected so that on application of a predetermined voltage across the switch, known as the breakdown voltage, a glow discharge occurs between the switch electrodes. The heat of the glow discharge, raises the temperature of the thermally responsive electrode which bends or flexes with increasing temperature. The electrodes are so positioned in the switch that the bending of the thermally responsive electrode brings. both the electrodes into direct physical and electrical contact to increase the current carrying capacity of the switch. The glow discharge path between the. electrodes is shunted when the electrodes are. in direct electrical connection, so. that the thermally responsive electrode is no longer heated by the glow discharge. After-the thermally responsive electrode has cooledsufficiently, it mpves toward its original spaced position with respect to the switch electrode cooperating therewitl1.and-in so. doing breaks the direct physical and electrical contact: between, the said electrodes. The switch is thenready: for another cycle of operation.

Such switches are. used extensively for starting purposes in alternating, current. circuits for electricv dis charge, double ended lamps having electrodes which re quire preheating. in suchcircuits the. switch is. connectcd in shunt to the, discharge path between the spaced electrodes of the lamp and on energization ofthe circuit connects the lamp electrodes in series with each other across a powersource during starting of the lamp-for a time sufficient toeffectpreheating of the lamp electrodes to a temperature such that the said electrodes are: ca-= pable of supporting a discharge therebetween withoutexcessive sputtering of electrode material. After the lamp electrodes have been soheated, the switch electrodes separate-from each other and an inductance connected operation produces a high transient or kick voltage; 60

in series with the lamp to serve as aballast duringlamp which is impressed across the lamp to start the.:latter immediately on breaking of the contact between the switch electrodes.

In order for the switch to function effectively .inisucii. a circuit, the breakdown voltage thereof, that is, age required to start the glow discharge between the spaced switchelectrodes, must be lower than. age applied across the higher than the voltage. lamp starts. of the transient voltage kick produced by the inductance applied thereacross after the.

the voltswitch before the. lamp starts and:

The switch. should not dissipate so .rrnuch.

lamp prematurely, that is, within one-tenth onbreaking'of the contact between the switch electrodes as would lower the transient voltage impressed across the lamp below that required to start the lamp. The current carrying capacity of the switch during opera tion ofth e glow discharge therein should be sufficient to generate enough heat to quickly raise the temperature of the thermally responsive electrode and to thus shorten the time required to start the lamp.

A common form of lamp ballast circuit in which such switches'are' used is an autotransformer type, two-lamp circuit in which a single ballast includes individual currentlimiting'devices for each of the lamps. The current limiting devices of the ballast are included in two branches of the circuit, one for each lamp, of the branches the current limiting device is an induct ance to ballast one of the lamps by inductive reactance only; The other branch of the circuit has a series connected inductance and capacitance, usually a condenser, to ballast the other of said lamps by inductive and catpacitive reactances. In the latter branch the electrical characteristics of the inductance and the capacitance are usually'such that the reactance due to the capacitance is'approximately 1.4 to 2.3 times the reactance due to the inductance.

The lamp ballas'ted by the inductive reactanee only is known as the lagging lamp, because the current in this branch of the two-lamp. circuit lags behind the voltage with a resultant lagging power factor; whereas, the lamp ballasted by both an inductive and a capacitive reactance is .known as the leading lamp, because the current in this branch of the circuit leads the voltage with a resultant leading power factor.

A thermal switch, usually of the glow discharge type, is'iconnected across each lamp in the manner and for the purpose stated above. When a switch of a lamp in the circuit opens after the preheating of the lamp electrodes, a transient voltage surge produced bythe inductance in the branch of the circuit including that lamp isimpressed across the lamp to start the main discharge between the preheated lamp electrodes.

.A well-known phenomenon in such lead-lag, switch start circuits is that the leading lamp has a much short er life than the lagging lamp. The main factor causing the shorter life-of the leading lamp is the starting of this of a second after energization of the two-lamp circuit andbe'fore the, lamp electrodes have attained their main discharge supporting temperature which requires at least threefourths of a second. This premature starting of the leading lamp-most frequently happens when the line'voltage is'bigh orwhen the lamps are well-aged and' have'become easy starters. It takes place before'the switch electrodes have made contact with each other. Execssiveballast peak voltages or a transient voltage surge may cause such premature starting. These occur in' commercial leading power factor circuits including a capacitance on closing of the manuallycontrolled switch, that'is, the line switch or wall switch, connecting the two lamp=circuit across the power supply line.

The'principal object of a the present invention is to provide a glow type thermal switch which increases the useful life in leading power factor circuits of discharge lamps having electrodes which require preheating.

Another object of the invention is to provide-a glow type thermal switch which is useful in'both branches of present'alternating current, lead-lag, switch-start two-lamp circuits and'which suppresses the tendency of the leading lamp to. startprernaturely onenergizi'ng of such circuits to'thereby increase the useful'life of lamps having elec trodes which require preheating. A furtherobject'of the invention is to provide such a switch useful for preand in one hea set venting the premature starting of lamps having preheatable electrodes and connected into a leading power factor v circuit, whether such circuit is included in lead-lag, two lamp circuits or not. Further objects and advantages of the invention will appear from the accompanying drawing illustrating a species of the invention, from the following detailed description of species of the invention and from the appended claims.

Features of the glow type thermal switch embodying the invention are a pair of electrically symmetrical electrodes and an ionizable gaseous filling of a mixture of specific gases at a total pressure within a range found to be effective for the purposes of the invention. 7

Electrically symmetrical electrodes have approximately equal electron-emissive areas, so that the breakdown voltage of the glow switch is normally equal in both directions of current flow of an alternating current source.

The total pressure of the gaseous filling in the switch, the relative amount and the kind of each of the components of the gaseous filling, and the electrical characteristics of the switch electrodes are so correlated in the switch of the present invention as to effectively reduce the transient voltage surge and excessive ballast peak voltages characteristic of leading power factor alternating current circuits of the above type on closure of the line switch controlling energization of the circuit, while retaining the other electrical characteristics of the switch necessary for its proper functioning in both lead and lag circuits.

I have discovered that the use of electrically symmetrical electrodes in the glow switch as distinguished from electrically asymmetrical electrodes, which are commonly used commercially in such switches, eleminates a voltage doubling effect caused by rectification of the current by switches with electrically asymmetrical electrodes. The current rectification characteristics of prior switches with asymmetrical electrodes causes a build up of charge on the capacitor in leading power factor circuits. Elimination of such current rectification by the use of electrically symmetrical switch electrodes thus reduces substantially the transient voltage surge occurring in the lead circuit on closure of the line switch.

The use of symmetrical switch electrodes does not, however, completely solve the problem of the premature starting of lamps of the above type in leading power factor circuits. 1 have demonstrated that the best results are obtained when this feature is combined with a gaseous filling in the switch of a major proportion of neon and a minor proportion of argon in certain percentages and at a total pressure for the mixture of gases within a particular range of total pressure. By proper correlation of the total pressure of the gas mixture filling, the relative percentages of neon and argon in the gaseous filling and by the use of symmetrical electrodes in the switch, I have succeeded in retaining the electrical characteristics of the starting switch necessary for its proper functioning in both leading and lagging power factor circuits while reducing the premature starting of such lamps in commercial leading power factor circuits in which premature starting is caused by transient voltage surges or excessive ballast peak voltages. Switches of the invention are successful in suppressing premature starting to the extent that the average life of the leading lamps in lead-lag, switch-start two-lamp circuits is about 90 per cent or better of the average life of the lagging lamps in such circuits. Prior to the present invention, the

leading lamps had an average life at best of about 65 per;

cent of the average life of the lagging lamps.

In the drawing accompanying and forming part of this specification:

Fig. l is a general diagrammatic representation of the wiring connections for a lad-lag, switch-start, two-lamp circuit including glow-type thermal switches, represented by symbols, connected in shunt to the discharge path of each of the elongated fluorescent lamps, which lamps are shown schematically;

Fig. 2 is a front elevational view of the essential parts of a species of glow-type thermal switch embodying the invention and useful in both branches of the circuit shown in Fig. 1;

Fig. 3 is a side elevational view of the switch shown in Fig. 2;

Fig. 4 is a front elevational view of a pair of electrodes of different structure from that of the electrodes shown in Figs. 2 and 3 and useful in the switch of Figs. 2 and 3;

Fig. 5 is a side elevational view of the electrodes shown in Fig. 4; and

Fig. 6 is a plot of relative starting transient voltages versus the percentage of argon in a mixture of neon and argon constituting the ionizable gaseous filling of switches embodying the invention and used in circuits of the type shown in Fig. 1.

Like numbers denote like parts in all the figures.

Referring to Fig. 1 of the drawing, the circuit illustrated is a two-lamp, lead-lag switch-start circuit particularly suitable for standard 40-watt fluorescent lamps 48 inches long and 1 /2 inches in diameter, and in the following description reference is made to such lamps, although it will be appreciated that the use of the glow switches embodying the invention is not limited to such circuits. The new switches are effective for starting both lamps in such circuit and for increasing the useful average life of the leading power factor lamps.

The electric discharge fluorescent lamps 1 and 2 illustrated schematically in Fig. l of the drawing are of identical structure and each comprises an elongated glass envelope in and 21!, respectively, having sealed into its opposite ends a pair of cooperating main discharge supporting filamentary electrodes 1b, 1e and 2b, 2e, respectively, and having a phosphor powder coating 1d and 212',- respectively, on its inner surface. The en velope of each lamp is filled with a starting gas, such as argon, at about 3 to 4 millimeters pressure. A small measured amount of mercury is contained 'in the envelope of each lamp and the vapor thereof is at a prescapacitor 8 and secondary winding 12 connected in series.

sure of about 10 microns during normal operation of the lamp and is highly emissive of 2537 A. wave length radiations. The phosphor emits visible light under excitation by the 2537 A. wave length radiations of the mercury vapor discharge.

The electrodes 1b, 1e and 2b, 2c, of the lamps 1 and 2 consist of coiled tungsten filaments supporting highly electron-emissive material, such as the oxides of barium and strontium and mixtures thereof. The electrodes are of the type which require preheating to a highly electron-emissive, main discharge supporting tem perature before starting the main discharge between the electrodes at the ends of the lamp envelope to obtain maximum useful operating life of the lamps.

The circuit for starting and operating the lamps 1 and 2 comprises an autotransformer 3 to which the lamps 1 and 2 are connected in the following manner: The lamp 1 is connected across the leading power factor load circuit 4, 5, whereas the lamp 2 is connected across the lagging power factor load circuit/i, 6. The load circuits are energized by the high reactance autotransformer 3 asfollows: Lead circuit 4, 5 includes primary windings, secondary winding 9 and capacitor 10 connected in series. Shunting resistor 11 is provided for 10. Lag circuit 4, 6 includes primary winding The three windings 8, 9 and 12 are wound on a single core in the usual ballast.

In both the load circuits, the primary and the respec- -,tive secondary are connected in adding voltage relaside of the primary winding 3. Point 14, which is the -low side of primary 8, is connected to lamp electrodes 1b and 2b of the lamps 1 and 2, respectively, whereas 5, the high sides of the load, circuits, namely. conductors jand6, are connected to lamp electrodes 1e and 2e, respectively.

The conductors 15 and 16 in, the drawing represent the supply line, the conductor lfibeing' the'grounded side of the supply line and 'connecte'dto the low side of the primary 8. The control or wall switch 17 of the two-lamp circuit is connected into the lead 15 connecting the ungrounded side of the supply and the high side of the primary 8. The shunting, or electrode preheat'circuit of each of the lamps 1 and 2, includes a flow-type thermal switch 13 and a condenser 26 connected in shunt with the switch for suppression of radio interference. In commercial practice the switch 18 and the condenser 3.9 are housed in a single opaque metal container to form'a unit known as a glow switch starter. The shunting circuit of the leading lamp 1 includes also a'compensator winding 20 and a condenser 21 con nected across the winding 20 also for suppression of radio interference. i

The circuit shown in Fig. l and described above is a conventional two-lamp, switch-start, lead-lag circuit which is energized in most instances from a 115 to 120 volt, 60-cycle alternating current supply, one side of which is grounded, and which circuit has an open circuit voltage of approximately 210 to 220 volts in each of its branches.

The reactance due to the capacitance is approximately L4 to 2.3 times the inductive reactauce of the autotransforrner windings 8 and 9, so that the load circuit 4 and 5 operates on a leading power factor. The lamp 1 connected across this load circuit is known as the leading lamp. The load circuit 4 and 6 having no capacitance therein operates on a lagging power factor and the lamp 2 connected across this load circuit is known as the lagging lamp.

The general mode of starting the lamp with this circuit arrangement is as follows: When the manual'control switch 17 is closed to energize the two-lamp circuit the glow switches 18. permit flow of heating current through the circuits connecting the electrodes 11'), 1e and 2b, 2e of the respective lamps in series. After a time sufficient to preheat the electrodes toa temperature whereat the electron emission of the electrodes is adequate to support a discharge between thev electrodes without excessive sputtering of the electrode material, which sputtering is the principal cause of short-lived lamps equipped with preheated electrodes, the switches open to break the series connection between the electrodes of each lamp and the resultant transient voltage surge or voltage kick produced on breakage of theshunt circuit by the windings 8, 9 and 10, which serve as inductances, is impressed across the spaced electrodes in the respective lamps 1 and 2 to initiatea mainpositive column discharge between said lamp electrodes and thus to start the lamps.

In accordance with the present invention, each of the switches 18 are of the same structure and the structure thereof is such that switch 18 included in the shunt, or electrode preheat, circuit of the leading lamp 1 effectively minimizes the transient voltage surge and excessive ballast peak voltages applied across the leading lamp 1 on closure of the wall switch 17 in prior leading power factor circuits including commercial starter witches of different structure. By minimizing, such voltage surges and excessive ballast peak, voltages the tendency of the leading lamp 1 to start before its electrodes 1b and 1e have been heated to a main discharge supporting temperature is reduced and the life of such lamps is increased correspondingly.

A preferred structure of switch 18 is shown in Figs. 2 and 3 of the drawing and comprises, a hermetically sealed glass envelope 23 having a re-entrant stern through the press24 of which a pair of current leading-in wires are'hermetically fused. Anexhaust tubulation125 is pro vided on the stem and through which the envelope 23 is first exhausted of air. and. then filled with ionizable gas befO1ie.,th e exhaust tribulation 25 is sealedofllduring manufacture, of the, switch. Theinner end parts, or terminations 26 and 27 of the, respective leading-in wires are spaced apart in the envelope 23 and preferably consist of nickel, or nickel-plated iron. The parts 28 and 29 of said leading-in wireshermetically fused with the glass stem 24 preferably consist of a copperclad nickel-iron alloy wire and the externally extending parts 30 and 31 th ere'of are of copper. T he said leadingwireparts are welded to each other.

The electrodes 32 and 33 of the glow, switch 18, each comprise a thermally responsive bimetallic strip. Each of'thestrips isl'curv'ed at one end as shown at 34 and 35, respectively. Before the end of each strip is bent, the augles at the corners of the strip connected by the longest diagonal are each approximately 75 degrees and the length along the longitudinal, sides thereof is about 0.325 inch. After theendof eachfstrip is bent, the projected length along the longitudinal sides. of the strip is, about 0.316 inch and .the projected depth of the bent endportion is about 0.035 inch. The bimetallic. strips are each 0.075 inch wide and 0.006.in thickness.

The bimetallic strip electrodes 32 and 33 are attached, preferably by spot welding, at theirflat ends to the spaced inner ends 26 and 27, respectively, of the leading-in wires, with the curved endsv 34 and35'of the electrodes diverging outwardly fromeachother as shown .in Fig.3. The fiat ends of the electrodes 32 and 33 are attached to diametrically opposite sides of the respective leading-in wire ends 26 and 27 in such manner that, at room temperature, the flat sides of the electrodes are in spaced-apart planes which are parallel with and, include therebetween the com: mon plane of said leading-in wire ends 26 and 27 The elongated strip electrodes32 and 33 extend in from said spaced leading-in wire ends 26 and 27 into normally spaced-apart, overlapping portions at their free ends as, shown in Fig. 2. The electrodes thusconf ont each other at their overlapping free ends for. intereng lgfiment at said ends when heated by a glow discharge.therebetween. The said confronting electrode ends are spaced apart whenat roorntemperature a distanceof approximately 0.025 to 0 0 v The curved ends 34 and 35 of there spectiveielectrodes 32 and 33 prevent the electrodes fromsticking together after the glow discharge between the electrodeshas been extinguished by. the intererigagement of the confronting end portions thereof and the electrodes have cooled down sufficiently to separate and return to their normally spaced apart' positions. Any tendency of the electrodes to stick together at that time is highly undesirable, becausev the discharge lamp across which the switch is connected cannot start until the switch electrodes separate from each other to break the direct series connection of the lamp electrodes through the said switch electrodes.

The bimetallic strip of each of. the electrodes 32 and 33 comprises two strips of metal having different coefiicients of lineareapansion. The metal strips are fastened togother as by welding and in such manner that the composite strip bends with changes in the temperature thereof. The electrodes are mounted with their sides of lower expansion coefficientfacing each other, so that the overlapping free ends of the electrodes 32 and 33 engageeach other when the electrodes bend in response to an increase in. the temperature thereof caused by the, heat generated by a glow discharge between the electrodes 32 and .33. The said electrode ends separate on cooling of the electrodes. While I prefer to make the sides of lower expansion of nickel iron alloy and the side of higher expansion of chrome iron, it will be understood that other suitable metals or alloys may be used.

The breakdown voltage of glow discharge type thermal switches is higher in the darkfthan in the light. This is known as the dark. effect of -such. switches and maybe minimized or completely eliminated by including a radioactive material in the switch envelope. In the switch 18 shown in Figs. 2 and 3, a coating 36 of uranium oxide is provided on the inner surface of the end of the glass envelope 23 of the switch so that the breakdown voltage of the switch 18 is approximately the same in the dark and in the light.

The electrodes 32 and 33 are coated with zinc to lower the breakdown voltage of the switch. The coated electrodes are of equal emitting area and act as electrically symmetrical electrodes during operation of the glow discharge in the switch. Rectification of the current passing through the switch and the attendant or consequent build up of excessive charge in the capacitor, such as the capacitor 10 of Fig. 1, used in leading power factor circuits is thus eifectively minimized by the switch 18 provided with electrically symmetrical electrodes.

The ionizable gaseous atmosphere in the switch 18 for conducting the glow discharge current between the electrodes 32 and 33 is constituted by a mixture of a major proportion of neon and a minor proportion of argon at a pressure within a range of about 45 to 70 millimeters, inclusive, at a room temperature of about 25 C. A pres sure within a range of about 50 to 60 millimeters, inclusive, at about 25 C, is highly effective and the best results are obtained when the pressure of the gaseous mixture is 53 millimeters at 25 C.

At pressures within these ranges the percentage of argon in the neon-argon gaseous filling of the switch envelope 23 is of critical importance in obtaining the advantages of the invention, and I have discovered that the best results are obtained with a mixture of neon with 10 to 20 per cent argon by volume. In the embodiment mentioned above, wherein the pressure of the gaseous mixture is 53 millimeters at 25 C., a'mixture of neon with 16 per cent argon is preferred.

The switch electrodes 37 and 38 shown in Figs. 4 and of the drawing are of equal emitting area to act as electrically symmetrical electrodes and are useful in place of the electrodes 32 and 32 in the switch 18 for obtaining the advantages of the invention. The electrodes 37 and 38 are mounted in the switch envelope 23 in the same manner as the electrodes 32 and 33 and also consist of zinc coated bimetallic strips of the same composition as the electrodes 32 and 33. The electrodes 37 and 38 are not curved at their free overlapping ends, however, as are the electrodes 32 and 33.

I have observed that the straight or flat-ended bimetallic strip electrodes 37 and 38 of Figs. 4 and 5 do not stick together at their overlapping ends after making contact with each other at said ends during operation of the switch 18 when the thickness of the strip constituting each electrode is approximately 0.008 inch. The increase in thickness from 0.006inch, the thickness of the strip electrodes 32 and 33having bent free ends, to 0.998 inch results in a very much higher return torque as the temperature of the bimetallic electrodes drops after extinguishment of the heat generating glow discharge therebetween. The increase in the strength of the return torque is sufficie'nt to overcome any tendency of the fiat, straight overlapping ends of the electrodes 37 and 33 to stick together in the operation of the switch.

I have found, however, that increasing the thickness of the bimetallic strip electrodes causes a corresponding increase in the tendency-of the leading lamp l to start prematurely in the circuit of Fig. 1. This tendency is overcome, in accordance with the present invention, by increasing the pressure of the preferred filling gas mixture of. 16 per cent argon, 84 per cent neon of the switch 18' from 53 millimeters to approximately 60 millimeters at about25 C. when the electrodes 37 and 33 are used ir place of the electrodes 32 and 33.

The fiat strip electrodes 37 and 38 are 0.075 inch wide andthe longitudinal sides thereof are approximately 0.300 inch long. The angles bisected by the longest diagonal of each strip electrode 37 and 38 are each about 75 C.

The discharge gap or the space between the switch elect trodes 37 and 38 at room temperature is the same as the space between the electrodes 32 and 33, that is, approximately 0.025 to 0.030 inch, inclusive.

The suppression of premature starting of the leading lamp 1 by use of the glow switch 18 equipped with either type of electrodes described above has been obtained without sacrificing the other electrical characteristics of the switch necessary for successful operation in both the lead and lag branches of the lead-lag two-lamp circuit. For cxampic, the voltage required to start the glow discharge between the spaced electrodes in the switch is approximately 135 volts, which is lower than the approximately 210 to 220 volts applied across the lamp in such circuits before the discharge in the lamp starts and is higher than the lamp operating voltage of approximately to volts.

The time required for the switch electrodes to engage each other and connect the lamp electrodes in series directly therethrough, known as the closingtime of the switch; the time the switch electrodes remain engaged or closed to effect preheating of the series connected lamp electrodes, known as the preheat time, and the time which elapses between the closing of the manual control switch 1'7 and the application of the transient or kick voltage applied across the lamp by the inductance on opening the glow switch, that is, when the glow switch electrodes sepa rate from each other to break the direct series connection therethrough of the lamp electrodes, and known as the lamp starting time, are within the established and acceptable limits in the art. For example, the lamp starting time, which includes a complete cycle of operation of the switch, is less than 10 seconds on the average.

An important requirement for such switches is that they must not dissipate therein so much of the transient starting voltage surge produced by the inductance on opening of the switch electrodes as to cause failure of the lamp to start on the voltage available across the lamp terminals. In other words, the switch must not reduce the kick voltage of the inductance below a critical value. In the above-described circuit employing the switches 18 of the present invention the kick voltage in both branches is approximately 650 volts which is highly effective for starting, the lamps.

In Fig. 6 of the drawing, the etlect of different percentages of argon in neon on the relative starting voltages is shown. The preferred range is 10 to 20 per cent of argon in neon, by volume, which is effective for obtaining a kick voltage of more than 600 volts for successfully starting the lamps. As far as I am aware, this relationship has not previously been known, and its existence provides a basis for glow switch design not previously attainable. iure argon gas is inherently oscillatory in character and causes transients of sufficient magnitude to contribute significantly to instant starting. Neon gas with a small percentage of argon does not have the same oscillatory characteristic as pure argon. However, while the use of neon and symmetrical electrodes would reduce instant starting substantially, a'considerable proportion of lamps would still instantly start with no starter at all. But by increasing the pressure of the argon-neon gas to the stated values, the current carrying capacity of the starter is sufiicieut to clip the voltage peaks appiied to the lamp before it starts. Neon gas with small percentages of argon at the relatively high pressures stated seems to have the ability to break down before the lamp can instantly start, and then keeps the voltage peaks applied to the lamp low enough to suppress instant starting and still supplies sufficient transient voltage to provide reliable starting when the switch opens.

It will thus be seen that glow-type thermal switches of the novel structure disclosed above and recited in the appended claims successfully meets the seemingly inconsistent requirements of suppressing transient voltage peaks and excessive ballast peak voltages tending to start prematurely discharge lamps having preheated electrodes in leading power factor circuits while avoiding dissipation in the switch of so much of the transient voltage surge applied across the lamp by the inductance on opening of the switch as would cause failure of the lamp to start.

Glow switches embodying the present invention successively meet these requirements and are therefore useful in both leading and lagging power factor circuits for lamp starting purposes and when used in leading power factor circuits are effective for increasing the useful operating life of the lamps used in such circuits to about 90 per cent or better, on the average, of the life of lamps used in lagging power factor circuits. This is an important advance in the art and makes economically feasible for the first time the replacement on a group replacement basis of lamps operating in lead-lag circuits in large lighting installations including both leading and lagging lamps in substantial numbers to thereby reduce the cost of maintenance of such installations.

While I have shown and described certain species of my invention, it will be understood that I contemplate that changes in the form and details of the switches shown and described may be made by those skilled in the art without departing from the spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A glow type thermal switch comprising a sealed envelope containing cooperating electrically symmetrical electrodes at least one of which is deformable by heat into engagement with another of said electrodes, and an ionizable gaseous atmosphere in said envelope comprising a mixture of neon with to per cent argon, by volume, at a total pressure of about 45 to 70 millimeters.

2. A glow type thermal switch comprising a sealed envelope containing cooperating electrically symmetrical electrodes at least one of which is deformable by heat into engagement with another of said electrodes, and an ionizable gaseous atmosphere in said envelope comprising a mixture of neon with 10 to 20 per cent argon, by volume, at a total pressure of about 50 to 60 millimeters.

3. A glow type thermal switch comprising a sealed envelope containing cooperating electrically symmetrical electrodes at least one of which is deformable by heat into engagement with another of said electrodes, and an ionizable gaseous atmosphere in said envelope comprising a mixture of neon with about 16 per cent argon, by volume, at a total pressure of about 53 millimeters.

4. A glow type thermal switch comprising a sealed envelope containing cooperating electrically symmetrical electrodes at least one of which is deformable by heat into engagement with another of said electrodes, and an ionizable gaseous atmosphere in said envelope comprising a mixture of neon with about 16 per cent argon, by volume, at a total pressure of about 60 millimeters.

5. A glow type thermal switch comprising a sealed envelope containing an ionizable gaseous atmosphere, a pair of cooperating normally spaced apart thermally responsive electrically symmetrical discharge supporting bimetallic strip electrodes anchored at one end in said envelope and having free ends confronting each other for interengagement, the terminations of the said confronting ends of said strip electrodes being curved outwardly from each other and presenting cylindrical anti-sticking contact surfaces, the said gaseous atmosphere in said envelope comprising a mixture of neon with 10 to 20 per cent argon, by volume, at a total pressure of about 45 to 70 millimeters.

6. A glow type thermal switch comprising a sealed envelope containing an ionizable gaseous atmosphere, a pair of cooperating normally spaced apart thermally re sponsive electrically symmetrical discharge supporting bimetallic strip electrodes anchored at one end in said envelope and having free ends confronting each other for interengagement, the terminations of the said confronting ends of said strip electrodes being curved outwardly from each other and presenting cylindrical anti-sticking contact surfaces, the said gaseous atmosphere in said envelope comprising a mixture of neon with 10 to 20 per cent argon, by volume, at a total pressure of about 50 to 60 millimeters.

7. A glow type thermal switch comprising a sealed envelope containing an ionizable gaseous atmosphere comprising a mixture of neon with 16 per cent argon, by volume, at a total pressure of about 53 millimeters, a pair of current inlead wires extending through the wall of and terminating in spaced relation within said envelope, a pair of electrically symmetrical thermally responsive bimetallic strip electrodes about 0.006 inch thick attached at one end to said inlead terminations within said envelope and having free ends confronting each other for interengagement, a coating of zinc on said electrodes, said electrodes being supported by said inlead terminations in spaced planes parallel with and including therebetween the common plane of said inlead terminations, the terminations of said confronting free ends of said strip electrodes being curved outwardly from each other and presenting cylindrical anti-sticking contact surfaces.

8. A glow type thermal switch comprising a sealed envelope containing a pair of electrically symmetrical thermally responsive bimetallic flat strip electrodes mounted in normally spaced apart positions in said envelope with a flat end of each of said electrodes confronting a corresponding end of the other, said electrodes being deformable by heat into interengagement at said confronting fiat ends and an ionizable gaseous atmosphere in said envelope comprising a mixture of neon with 10 to 20 per cent argon, by volume, at a total pressure of about 45 to 70 millimeters.

9. A glow type thermal switch comprising a sealed envelope containing a pair of electrically symmetrical thermally responsive bimetallic flat strip electrodes mounted in normally spaced apart positions in said envelope with a fiat end of each of said electrodes confronting a corresponding end of the other, said electrodes being deformable by heat into interengagement at said confronting flat ends and an ionizable gaseous atmosphere in said envelope comprising a mixture of neon with 10 to 20 per cent argon, by volume, at a total pressure of about 50 to 60 millimeters.

10. A glow type thermal switch comprising a sealed envelope containing an ionizable gaseous atmosphere comprising a mixture of neon with about 16 per cent argon, by volume, at a total pressure of about 60 millimeters, a pair of current inlead wires extending through a wall of and terminating in spaced relation within said envelope, a pair of electrically symmetrical thermally responsive bimetallic flat strip electrodes about 0.008 inch in thickness attached at one end to said inlead terminations within said envelope and having free flat ends confronting each other for interengagement, a coating of zinc on said electrodes, said electrodes being supported by said inleads in spaced planes parallel with and including therebetween the common plane of said inlead terminations.

References Cited in the file of this patent UNITED STATES PATENTS 2,242,902 Chirelstein May 20, 1941 2,332,809 Peters Oct. 26, 1943 2,333,710 Deimel Nov. 9, 1943 2,457,487 Peacock et a1. Dec. 28, 1948 2,650,278 Foulke Aug. 25, 1953 2,673,266 Bjorkman Mar. 23, 1954 

1. A GLOW TYPE THERMAL SWITCH COMPRISING A SEALED ENVELOPE CONTAINING COOPERATING ELECTRICALLY SYMMETRICAL ELECTRODES AT LEAST ONE OF WHICH IS DEFORMABLE BY HEAT INTO ENGAGEMENT WITH ANOTHER OF SAID ELECTRODES, AND AN IONIZABLE GASEOUS ATMOSPHERE IN SAID ENVELOP COMPRISING A MIXTURE OF NEON WITH 10 TO 20 PER CENT ARGON, BY VOLUME, AT A TOTAL PRESSURE OF ABOUT 45 TO 70 MILLIMETERS. 